The output voltage of a generator may be regulated by comparing the voltage at a point of regulation (POR) with a reference voltage and using a voltage regulator to maintain the output voltage at a desired level. One failure mode for such a generator control may be referred to as a “loss of voltage sensing” failure mode. In this failure mode, the voltage regulator detects a voltage of 0 (or a very low voltage) and attempts to increase the generator output to raise the voltage back to the desired level. However, since in fact no feedback is being provided to the voltage regulator, the voltage regulator will continue to increase the generator output, and the generator output voltage will quickly reach dangerously high levels. This may damage equipment connected to the generator. The cause of this failure mode may be, for example, a loose wire providing the voltage feedback to the voltage regulator or a short in the voltage regulating circuit.
This failure mode can cause problems in constant frequency electrical systems. However, the severity of this failure mode is even greater in variable frequency electrical systems such as those sometimes used on aircraft. Such variable frequency electrical systems are becoming increasingly popular because of their overall lighter weight and increased efficiency.
In a variable frequency system, the generator may operate at frequencies nearly twice as high as the frequencies used in constant frequency systems. The higher the frequency at which a generator operates, the shorter the time it will take to reach an overvoltage condition. Therefore, effective protection against this failure mode in variable frequency systems is becoming an important concern.
Some systems now require generators that limit overvoltage to about 150 V rms for 115V AC electrical systems and to 300V rms for 230V AC electrical systems. A conventional approach to overvoltage protection is to monitor voltage levels and disconnect the generator from the power supply when an overvoltage is detected. This approach, however, is too slow to provide effective protection for the above failure mode, especially in a variable frequency system. It is therefore desirable to provide a method and apparatus for addressing this failure mode in a manner that limits generator overvoltage and protects components connected to a generator.